Hi Nozmo, what you are experiencing is an age old issue between markup code/css and browsers. With a variety of browsers from different makers, and lacking a true standardization in markup and CSS the only known solution is to design according to how well you can make a round peg fit into a square hole.
IE tells you if there are errors on any web page it displays with a yellow triangle and an exclamation mark in it at the bottom of your browser. This for the most part is just a minor FYI tool that a page has some erroneous code and can't be displayed properly. Again, what is "properly" if there is no standard "proper"? So..I always tell folks, to use the minimalist design theory and keep the code short, sweet, basic, and functional, for both markup and CSS.
If you feel you want to take on a challenge, you can make the changes yourself to the Ocean Mist theme, but if you are not fully prepared to learn a lot, and are willing to work with generally "popular" ideas of how a web page should display and the code that is best (by opinion), then I would say have fun while doing it.
The issue is not so daunting. The W3C will validate your code based on "widely accepted" and proper use of code, but again, that doesn't mean the browser makers are forced to follow suit and "make use" of the widely accepted code usage.
I have always come down to designing a web site based purely on what I think the most popular browser will be used to view it, which varies with a target market, type of content of the website, and type of computer the viewer might have (mac vs IBM PC vs linux/unix). There are so many variables. A good rule of thumb is to have more than one browser installed, and design or code your site to display as good as possible across many browser platforms, or you could pick one browser and make it almost perfect for that one browser.
You shouldn't have a problem making the corrections noted in red if you cut and paste. Keep in mind, the most common problem from attempting to fix is a simple typo, and also keep in mind, an erroneous "space" is a typo too.